Multi-group transmissions consist of two or more transmission groups, usually arranged in series, by combining the groups a large number of gears can be produced. Increasingly, they are designed as automated gearshift transmissions consisting, for example of an input group, a main group and a downstream range group. Such transmissions are used particularly in utility vehicles since they provide an especially fine gradation of gears, for example 12 or 16 gears, and are highly efficient. For a smaller number of gears, configurations with only a main group and an input group or a main group and a range group are also possible. Furthermore, compared with manual gearshift transmissions they are characterized by a high degree of operating comfort and, compared with automatic transmissions, their production and operating costs are particularly economical.
By virtue of their structure conventional multi-group gearshift transmissions, like all manual or automated gearshift transmissions not shifted under load, undergo a traction force interruption during gearshifts since the force flow from the drive motor is always interrupted by disengaging a clutch to disengage the engaged gear without load, to synchronize the transmission and the drive motor in a neutral position to a connection speed, and then to engage the target gear. Since the vehicle is rolling during the traction force interruption, undesired speed increases or decreases can occur. In addition fuel consumption can increase. Whereas with passenger motor vehicles the traction force interruption, which affects the driving dynamics, is as a rule perceived only as annoying, for example during upshifts in a driving style of sporty orientation, in the case of heavy utility vehicles the driving speed can be reduced to the point where an upshift is made impossible and, on uphill stretches, undesired downshifts, creep-driving or even additional starting operations may be necessary.
From DE 10 2006 024 370 A1 by the present applicant a traction-force-supported automated multi-group transmission with a splitter group as its input or upstream transmission, a main group as its main transmission and a range group as its output or downstream transmission is known. The structure of this known multi-group transmission with its input transmission and the main transmission enables a direct gear to be engaged as an intermediate gear during a gear change. For this, a direct connection is temporarily formed between an input shaft of the input transmission and a main shaft of the main transmission by means of a change-under-load clutch. This renders the main transmission and the splitter group free from load, so that the engaged gear can be disengaged, the transmission synchronized and the target gear engaged. During this the change-under-load clutch transmits the motor torque to the transmission output, and a dynamic torque that is released during a speed reduction between the original and target gears is used to compensate the traction force interruption to a large extent. The change-under-load clutch can be positioned between the input transmission and the main transmission or between an ordinary starting clutch and the input clutch. The gear ratio of the intermediate gear is determined by the direct connection of the input shaft to the main shaft. Although the starting clutch can remain engaged during the gearshift operation, it is present as a separate starting element.
Furthermore, from EP 0 933 558 A1 a variable-speed transmission of a motor vehicle having a countershaft structure is known, in which one of the gears, preferably a 2nd gear, is arranged on an auxiliary shaft. To reduce the jerk caused by the traction force interruption during a gearshift operation, in addition to a conventional starting or separator clutch, a friction clutch is arranged on the transmission input, which acts upon this gear via the auxiliary shaft. The two clutches can for example be made as a double clutch, such that a transmission input shaft connected to the starting clutch is surrounded coaxially by a hollow shaft connected to the friction clutch. When actuated the friction clutch, via a drive input gearset, transmits motor torque to the auxiliary shaft, on which a gearwheel of the gear is mounted, which is engaged with a gearwheel on a transmission output shaft.
During a gearshift from an engaged gear to the auxiliary shaft gear, i.e. for example from the 1st to the 2nd gear, the friction clutch is engaged, i.e. the auxiliary gear is engaged. As a result of the speeds produced, the torque flow via the separator clutch is reduced so that the torque path of the gear engaged becomes load-free and the 1st gear can be disengaged. If now the transmission is in the auxiliary-shaft gear, then in the next gearshift, i.e. for example from the 2nd to the 3rd gear, the target gear is engaged while the separator clutch is disengaged and the separator clutch is then engaged, so that now the torque path of the auxiliary-shaft gear becomes load-free and the friction clutch can be disengaged again. All gearshifts in which the auxiliary-shaft gear is the original or the target gear can therefore be carried out under load and thus without traction force interruption. Furthermore, in gearshifts between gears that are directly adjacent to the auxiliary-shaft gear, the shift jerk can be reduced by operating the friction clutch in a slipping mode. For other gearshifts a traction force interruption still takes place in the conventional manner.
The transmission described in EP 0 933 558 A1 serves the purpose of improving the shifting comfort, in the lower gears which are particularly prone to jerky shifting, with the least possible cost and effort. In that respect it can fulfil its set objective completely. However, the traction force support is limited to certain gearshifts. But in the case of a multi-group transmission with a fine gradation and a large number of gears, traction force interruptions in all the traction upshifts and traction downshifts, i.e. in the higher gears as well, can have a disturbing effect. Besides, an additional auxiliary shaft and its drive input gearset demand a corresponding amount of structural space, which in group transmissions usually comprising two countershafts, is not always available or can only be made so with difficulty.